1. Field of the Invention
The present invention relates to in-situ methods of coal gasification, and more particularly but not by way of limitation, to methods of forming fractures in subterranean coal bearing formations by injecting liquified gases into at least one substantially horizontally disposed fracturing borehole drilled into the formation, and thereafter igniting the coal in the fractured formation and recovering the resulting hot gases via a producer well.
2. Brief Description of Related Art
Coal gasification is a process that converts coal from a solid to a product gas. Underground or subterranean coal gasification involves controlled conversion of coal to a combustible product gas containing methane, hydrogen, carbon monoxide, and carbon dioxide, with minor amounts of impurities. An underground or subterranean coal gasification process involves pumping an oxidant (air or oxygen) and steam down an injection well into a coal seam, igniting the coal and recovering the product gas resulting from combustion of the coal via a production well.
For nations, such as the United States, which have large coal resources and decreasing petroleum and natural gas reserves, the need for producing gas from coal increases. Several coal gasification processes have heretofore been employed. The most common process utilizes lump coal and a vertical retort. Air and coal are fed into the top of the retort and steam is introduced into the bottom of the retort. The air, gas and steam heat the coal and react with the coal to convert it to gas. When air and steam are used as the reacting gases, water gas is produced: whereas, when air and steam are used as the reacting gases, producer gas is produced.
Two additional commercial processes have been employed to gasify coal, namely the Winkler process and the Koppers-Totzek process. The Winkler process employs a fluidized bed in which powered coal is agitated with reactant gases, i.e., steam and oxygen. However, in the Koppers-Totzek process, which operates at a much higher temperature, the powered coal is reacted with steam and oxygen while it is entrained with the gases passing through the reactor. Each of the above-referenced process are used for fuel gas production and in the generation of gases for chemical and fertilizer production.
Numerous in-situ coal gasification processes to recover hydrocarbons from coal have heretofore been proposed. Such in-situ processes have generally encountered control problems and have not proven to be economically feasible. However, in-situ coal gasification represents a technology with considerable potential in power generation, industrial applications and petrochemical feedstocks for countries such which have vast coal deposits, as the United States.
Therefore, new and improved economical and commercially feasible processes for in-situ coal gasification are being sought which overcome various problems, including those described above. It is to such new and improved process that the present invention is directed.